The reason this takes a file path instead of a File object is that we can sidestep the issue of users providing File implementations instead of a dart:io file. Providing the path as a string makes it obvious this uses the real file system.

Having said that, it sort of kicks the can down the road to the framework to handle non dart:io Files. I don't think we have a way to detect that in Dart code, we'd need a native method that checked the Dart handle or similar. Maybe this should accept a Dart file so that we can check that and throw.

Thoughts @dnfield @zanderso ?

@dnfield pointed out we can check the file runtime type. That makes it work

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